January 23, 2007

Summer Heat History

We are sure by now you’ve heard the news that global warming is causing heat waves to increase in frequency, intensity, and duration around the world, humans are suffering and dying at alarming rates in the ever-increasing summer heat, and it could all be prevented if we seriously addressed the greenhouse issue. Just go on-line and look up heat waves and global warming. Within seconds we found global warming advocacy sites claiming that “Heat-waves in Europe are associated with significant morbidity and mortality. A preliminary analysis of the 2003 heat-wave in Europe estimated that it caused 14,802 excess deaths in France, 2045 excess deaths in the United Kingdom, 2099 in Portugal.” Or, try “An estimated 15,000 people died as a result of the heatwave in France last August. Chicago’s heatwave of July 1995 killed about 739.” If you’ve not seen enough, you will quickly find headlines like “Consequence: deadly heat waves and the spread of disease” in which you learn that “More frequent and more intensive heat waves could result in more heat-related deaths. These conditions could also aggravate local air quality problems, already afflicting more than 80 million Americans. Global warming is expected to increase the potential geographic range and virulence of tropical diseases as well.”

We have addressed the heat wave story many times at World Climate Report, and we have shown research results demonstrating that the American public is far less susceptible to heat waves than at any time in the past thanks to good old technology. The last we checked, two of the fastest growing cities in North America are Phoenix and Las Vegas where temperatures routinely exceed 110°F in the summer months. These cities seem stuck for months in what the rest of the world would call heat waves, and they have clearly engineered their way around uncomfortable conditions outdoors. A normal summer day in Phoenix or Las Vegas would certainly be a disaster in many other cities worldwide (of course, one inch of snow in Phoenix would be a disaster as well). And given the apparent fact, from the internet at least, that the buildup of greenhouses is causing heat waves to increase, it is tempting to blame all recent heat waves on the dreaded global warming phenomenon.

Well, the scientific literature contains an article in recent issue of the Journal of Climate that may call into question any claims that summer heat is worse now then ever, at least in central Europe. A team of scientists from the Swiss Federal Research Institute gathered 180 cores from larch trees in an attempt to reconstruct summer temperatures over a period greater than 1,000 years. Some of the cores came from trees still growing in Switzerland while other cores came from historic buildings in which the wood certainly would have come from nearby trees. Ultimately, the tree ring data allowed for a summertime reconstruction back to A.D. 755. They were able to calibrate the tree ring record to climate by using historical temperature data for the area from A.D. 1818 to near present. The article contains many pages of material showing how accurately they were able to reconstruct summer temperatures with the tree ring data – the team convincingly shows that the tree rings are very sensitive to summer temperatures and that the tree rings can be used to estimate summer temperatures back to A.D. 755.
Büntgen et al. write “The reconstruction indicates positive temperatures in the tenth and thirteenth century that resemble twentieth-century conditions, and are separated by a prolonged cooling from ~1350 to 1700.” Further, they note “Maximum temperature amplitude over the past 1250 yr is estimated to be 3.1°C between the warmest (1940s) and coldest (1810s) decades.” Also, the team writes “Warm summers seem to coincide with periods of high solar activity, and cold summers vice versa.”

With respect to the Medieval Warm Period (MWP) and the Little Ice Age (LIA), they find “For the MWP, significant interdecadal fluctuations are recorded, with high temperatures in the 960s–80s and 1200s–20s, and low temperatures in the 1040s–60s. The reconstruction shows strong interdecadal fluctuations through a generally cooler period between ~1350 and 1820, coinciding with the LIA. Low temperatures are recorded during 1580–1710, and relatively high temperatures during ~1500 and ~1800.”

OK – did you notice that since A.D. 755, the warmest decade occurred in the 1940s? As we see in the Figure 1, the summers of Switzerland cooled noticeably after the 1940s, and despite the great heat wave of 2003, the 20 year smoothed average is not back to conditions from 60 years ago. Furthermore, it is obvious that the current summer temperatures were equaled or exceeded many times in the past. There is no doubt that 2003 was a hot one, but so was A.D. 970.


Figure 1. (A) Alpine summer temperature reconstruction with the orange and blue boxes denoting the 10 warmest and coldest decades, respectively, and the smoothed red line being a 20-yr low-pass filter. Temperatures are expressed as anomalies from 1901–2000. (B) High-frequency comparison between the chronologies (red is from the Büntgen et al. (2006) study) and blue is from a prior study. The 51-yr moving correlations (black) indicate their temporal relationship, with the horizontal line denoting the 95% significance level. (C) Length fluctuation (m) and 50-yr average mass balance (gray) of the Great Aletsch glacier.

The reconstruction for Switzerland shows us a lot about the temperature history of the past 1,000+ years. We see warming and cooling occurring throughout the record, we easily see the Medieval Warm Period and the Little Ice Age, and we see a connection to output from the sun. The larch trees of Switzerland have seen a lot in the past 1,250 years, and recent conditions aren’t generally something that they haven’t seen many times before.

Reference:

Büntgen, U. D.C. Frank, D. Nievergelt, and J. Esper, 2006, Summer Temperature Variations in the European Alps, A.D. 755–2004. Journal of Climate, 19, 5606-5624.




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